[PATCH] powerpc/bpf: fix write protecting JIT code

[Michael Ellerman]

Daniel Borkmann <daniel at iogearbox.net> writes:
> On 10/25/21 8:15 AM, Naveen N. Rao wrote:
>> Hari Bathini wrote:
>>> Running program with bpf-to-bpf function calls results in data access
>>> exception (0x300) with the below call trace:
>>>
>>>     [c000000000113f28] bpf_int_jit_compile+0x238/0x750 (unreliable)
>>>     [c00000000037d2f8] bpf_check+0x2008/0x2710
>>>     [c000000000360050] bpf_prog_load+0xb00/0x13a0
>>>     [c000000000361d94] __sys_bpf+0x6f4/0x27c0
>>>     [c000000000363f0c] sys_bpf+0x2c/0x40
>>>     [c000000000032434] system_call_exception+0x164/0x330
>>>     [c00000000000c1e8] system_call_vectored_common+0xe8/0x278
>>>
>>> as bpf_int_jit_compile() tries writing to write protected JIT code
>>> location during the extra pass.
>>>
>>> Fix it by holding off write protection of JIT code until the extra
>>> pass, where branch target addresses fixup happens.
>>>
>>> Cc: stable at vger.kernel.org
>>> Fixes: 62e3d4210ac9 ("powerpc/bpf: Write protect JIT code")
>>> Signed-off-by: Hari Bathini <hbathini at linux.ibm.com>
>>> ---
>>>  arch/powerpc/net/bpf_jit_comp.c | 2 +-
>>>  1 file changed, 1 insertion(+), 1 deletion(-)
>> 
>> Thanks for the fix!
>> 
>> Reviewed-by: Naveen N. Rao <naveen.n.rao at linux.vnet.ibm.com>
>
> LGTM, I presume this fix will be routed via Michael.

Thanks for reviewing, I've picked it up.

> BPF selftests have plenty of BPF-to-BPF calls in there, too bad this was
> caught so late. :/

Yeah :/

STRICT_KERNEL_RWX is not on by default in all our defconfigs, so that's
probably why no one caught it.

I used to run the BPF selftests but they stopped building for me a while
back, I'll see if I can get them going again.

cheers